1. Field of the Invention
The present invention relates generally to a gas sensor element to be employed in a combustion control for an internal combustion engine mounted on automotive vehicles.
2. Description of the Related Art
Considering from a viewpoint of earth environmental protection, alternative fuel engine technologies have been studied and examined, such as direct injection gasoline engines and compresses natural gas (CNG) engines. Following this, gas sensor elements for use in such direct injection gasoline engines and CNG engines have also been attracted interest recently.
However, in the technological field of the direct injection gasoline engine, because the direct injection gasoline engine is different in combustion mechanism from other types of gasoline engines, the direct injection gasoline engine often generates an unburned fuel gas, and emits an exhaust gas including the unburned fuel gas when the direct injection gasoline engine starts and during the operation of the direct injection gasoline engine. Further, the CNG engines have a tendency to increase the amount of hydrogen gas involved in the exhaust gas by a different fuel specification when compared with a case of a conventional gasoline engine. Accordingly, there becomes a problem of causing an output shift of a gas sensor element from a correct output value.
It is considered that such a problem is generated by the difference in the passage speed between hydrogen gas and another combustion gas that are passing through a porous diffusion resistance layer. The porous diffusion resistance layer is capable of limiting the introduction or passage amount of target gases to be measured. That is, because hydrogen gas has a smaller molecular weight than that of another combustion gas such as oxygen gas in air, the hydrogen gas can arrive at or reach a target gas electrode in the gas sensor element faster than another combustion gas. This causes the concentration of hydrogen gas to be increased in the target gases around the target gas electrode. In other words, an excess amount of hydrogen gas becomes present around the target gas electrode. This would cause such an output shift of the gas sensor element from its correct output value.
In particular, an air-fuel sensor (A/F sensor) capable of detecting an air-fuel ratio by using a critical current has a tendency to cause a remarkable output shift from a correct output value thereof. That is, because the A/F sensor has a long diffusion length in the porous diffusion resistance layer, the difference of a diffusion rate between hydrogen gas and another combustion gas becomes large. As a result, such an A/F sensor has a large shift output from a correct output value.
There is a tendency to generate more hydrogen gas in unstable combustion conditions, for example, at the engine start. Thus, there becomes an important problem regarding the output shift of the gas sensor element from a correct output value. In addition, it is necessary as an important matter that an air-fuel ratio (A/F ratio) in an internal combustion engine is shifted to a specified ratio of an optimum combustion condition in order to obtain a superior purifying function as quickly as possible, simultaneously with the temperature rise of a catalyst converter (a three-way catalytic converter), mounted on an exhaust gas pipe, capable of purifying exhaust gas emitted from an internal combustion engine of a vehicle. That is, it is very important to activate or operate the gas sensor element and to provide the correct output of the gas sensor element without any output shift from the correct output value, as quickly as possible.
FIG. 8A is a sectional view of a catalyst support trap layer 92 in a gas sensor element 9 according to a related art, through which target gases such as hydrogen gas H2 and oxygen gas O2 to be measured are passing. FIG. 8B is a sectional view of the catalyst support trap layer 92 of the gas sensor element 9 according to the related art, and FIG. 8B shows that the target gases reach the target gas electrode in a measurement room. FIG. 9 is a detailed sectional view showing the catalyst support trap layer 92 in detail that is formed on the outer peripheral surface 920 of a porous diffusion resistance layer 912 in the gas sensor element 9 according to the related art. FIG. 10A shows components that form the catalyst support trap layer 92 in the gas sensor element 9 according to the related art before breaking its endurance capability (or durability). FIG. 10B shows the components that form the catalyst support trap layer 92 in the gas sensor element 9 according to the related art after breaking its endurance capability (or durability).
As shown in FIGS. 8A, 8B, 9, 10A, and 10B, such a related art technique has proposed the gas sensor element 9 having the catalyst support trap layer 92. For example, Japanese patent No. JP 3488818 and Japanese patent laid open publication No. JP 2002-181769 have disclosed such a related art technique.
The gas sensor element is composed mainly of a solid electrolyte body 913 of oxygen ionic conductivity, a target gas electrode 914, a reference gas electrode 915, a porous diffusion resistance layer 912, and a catalyst support trap layer 92. The target gas electrode 914 and the reference gas electrode 915 are formed on both the surfaces of the solid electrolyte body 913, respectively. The target gas electrode 914 is covered with the porous diffusion resistance layer 912 through which the target gas passes. The catalyst support trap layer 92 supports a noble metal catalyst 922 (see FIG. 9) on the outer peripheral surface of the porous diffusion resistance layer 912. A part of the hydrogen gas is burned with the noble metal catalyst 922 supported in the catalyst support trap layer 92, and it is as a result possible to suppress the quick reaching of the hydrogen gas to the target gas electrode 914.
However, in view of the requirement regarding a quick response (or rapid activation) of the gas sensor element and the environmental change during a high temperature use, there is a possibility of reducing the catalyst capability of the catalyst support trap layer 92 by cohering particles of the noble metal catalyst 922 involved in the catalyst support trap layer 92 in a high temperature environment, as shown in FIG. 10B. In this case, as shown in FIG. 8A and FIG. 8B, hydrogen gas H2 can pass through the porous diffusion resistance layer 912 faster than another combustion gas. As a result, the output shift from a correct output value occurs in the gas sensor element.
In addition, Japanese examined patent publication (after examination) No. JP S63-66448 has disclosed another related-art technique of the gas sensor element which has a first catalyst support trap layer and a second catalyst support trap layer where the first catalyst support trap layer is formed on the second catalyst support trap layer. Platinum (Pt), palladium (Pd), or an alloy of platinum (Pt) and palladium (Pd) is added, as a noble metal catalyst, to the first catalyst support trap layer. Rhodium (Rh), ruthenium (Ru) or an alloy of rhodium (Rh) and ruthenium (Ru) is added, as a noble metal catalyst, to the second catalyst support trap layer. However, if the noble metal catalyst includes less palladium (Pd), there causes a possibility of cohering platinum (Pt) and rhodium (Rh) in oxidation atmosphere. On the other hand, if the noble metal catalyst includes excess amount of palladium (Pd), there is a possibility of absorbing a specified gas in the target gases by the presence of palladium (Pd). This causes deterioration of quick response capability of the gas sensor element.